The present invention relates to a copy item transport device provided in an image forming apparatus such as a copying machine, a printer and like apparatus, and more particularly to a structure of a separating device for separating copy items while being transported and to a control for transporting of copy items.
Conventional copy item transport devices for use in image forming apparatuses to transport copy items from a cassette to a photosensitive member have been known to be provided with a separating device including a pair of separating rollers. Such separating device is arranged in a specified position along a transport path between the cassette and a pair of registration rollers so that the device may prevent a so-called multiple feeding that a plurality of copy items are simultaneously fed from the cassette.
FIG. 7 is a side elevation view in section showing a construction of one of the conventional separating devices.
In this figure, the separating device 100 is provided downstream from a cassette so as to transport a copy item to a pair of registration rollers arranged downstream of a item transport direction while separating multiply fed copy items.
The separating device 100 includes a forward roller 103 and a pair of reverse rollers 104, 104'. The forward roller 103 is fixedly mounted on an intermediate portion of a drive shaft 105 whose front and rear ends are supported rotatably respectively by front and rear support plates 101, 102. The drive shaft 105 is driven in a direction of transporting the copy item to the downstream side by an unillustrated drive mechanism (shown in the arrow direction in FIG. 7 and hereinafter referred to as forward direction). The forward roller 103 is provided closer to the front support plate 101 than to the rear support plate 102 in consideration of the arrangement of a transport path.
The reverse rollers 104, 104' are mounted on an intermediate portion of a drive shaft 106 rotatably respectively through friction clutches 107, 107'. The drive shaft 106 is supported rotatably at front and rear ends thereof respectively by the front and rear support plates 101, 102 and movable toward and away from the drive shaft 105. The friction clutches 107, 107' are provided in such a manner that a transmission of a drive torque of the drive shaft 106 to the reverse rollers 104, 104' is cut off to put the reverse rollers 104, 104' in an idly rotating state when the driven torque from the forward roller 103 is larger than a predetermined value (hereinafter referred to as torque limit value).
The reverse rollers 104, 104' are arranged equidistantly from a widthwise center O of the forward roller 103. Thus, a distance L1 between the front support plate 101 and a widthwise center A of the front reverse roller 104 is shorter than a distance L2 between the rear support plate 102 and a widthwise center B of the rear reverse roller 104'.
At respective support positions where the shaft 106 is supported by the front and rear support plates 101, 102 are provided helical springs 108, 108' to urge the drive shaft 106 toward the drive shaft 105. The springs 108, 108' are provided with the same spring constant to give substantially the same urging forces to the both end of the drive shaft 106. By the resilient forces of the springs 108, 108', respective circumferential surfaces of the reverse rollers 104, 104' are pressed against a circumferential surface of the forward roller 103 at a specified pressure. Further, the drive shaft 106 is driven in a direction opposite to the forward direction (hereinafter referred to as reverse direction) by an unillustrated drive mechanism.
The springs 108, 108' are provided with such a resilient force that the driven torque which is transmitted from the forward roller 103 to the reverse rollers 104, 104' by the frictional force between the forward roller 103 and the reverse rollers 104, 104' is a predetermined amount greater than the torque limit value of the friction clutches 107, 107'. Accordingly, in the state where the reverse rollers 104, 104' are in pressing contact with the forward roller 103, the reverse rollers 104, 104' are permitted to rotate in the forward direction together with the forward roller 103.
On the other hand, in the state where multiple copy items are nipped between the forward roller 103 and the reverse rollers 104, 104', the reverse rollers 104, 104' are rotated in the reverse direction by the drive shaft 106 owing to the fact that the multiple copy items cause the driven torque to be smaller than the torque limit value, and place the friction clutches 107, 107' in the engagement state.
Further, FIG. 8 shows an arrangement of a conventional copy item transport device including the above-mentioned separating device 100 and a pair of registration rollers. FIG. 9 is a timing chart showing a timing relationship between principal members of the conventional copy item transport device.
A registration sensor 111 for detecting the copy item transported to a pair 110 of registration rollers is arranged at an appropriate position on the downstream side of the separating device 100.
Upon printing start being instructed at time t1 in a stand-by state where the image forming apparatus is in an inoperative state, driving of the reverse rollers 104, 104' is started at time t2. Thereafter, driving of an unillustrated feed roller and the forward roller 103 is started at time t3 after lapse of a specified duration from time t2.
The forward roller 103 is maintained in an idly rotatable state during the duration from t2 to t3. A driven torque acting on the reverse rollers 104, 104' due to the frictional force between the forward roller 103 and the reverse rollers 104, 104' is smaller than the torque limit value of the friction clutches 107, 107'. Accordingly, the forward roller 103 is rotated in the reverse direction by the reverse rollers 104, 104'.
When the forward roller 103 is started driving in the forward direction, i.e., in the arrow direction in FIG. 7, at time t3, the driven torque acting on the reverse rollers 104, 104' becomes larger than the torque limit value. Consequently, the reverse rollers 104, 104' are rotated in the forward direction by the forward roller 103.
The copy item is fed out from the cassette by driving the feed roller and then nipped between the forward roller 103 and the reverse rollers 104, 104' and further transported downstream by the rotational force of the rollers 103. At this time, if multiple copy items are being fed, the driven torque acting on the reverse rollers 104, 104' becomes smaller than the torque limit value of the friction clutches 107, 107'. Consequently, the friction clutches 107, 107' come into the engagement state and the reverse roller 104, 104' are rotated in the reverse direction by the drive shaft 106. The copy item in contact with the forward roller 103 is transported to the downstream side by the forward driving of the forward roller 103. On the other hand, the other copy item in contact with the reverse rollers 104, 104' is kept at the nip position of the separating device 100.
The copy item being transported to the downstream side is detected by the registration sensor 111. Upon lapse of a specified duration after detection of the presence of copy item, the driving of the feed roller is stopped, and the registration roller pair 110 is started driving at time t4. The registration roller pair 110 is driven for a short duration T (t6-t4) to assuredly nip a leading end of the copy item.
On the other hand, the driving of the forward roller 103 is stopped at a time t5 in the duration T, during which the registration roller pair 110 is kept being rotated, to ensure the separation of copy item. On the other hand, the reverse rollers 104, 104' are continuously rotated for a longer duration beyond time t7 at which the driving of the registration roller pair 110 is started again to transport the copy item to a photosensitive member.
However, there have been the following problems in the prior art copy item transport device. In the separating device 100, the distance L1 between the front support plate 101 supporting the front end of the shaft 106 and the front reverse roller 104 is smaller than the distance L2 between the rear support plate 102 supporting the rear end of the shaft 106 and the rear reverse roller 104'. However, the springs 108, 108' have the same resilient. Accordingly, a pressing force of the front reverse roller 104 against the forward roller 103 differs from a pressing force of the rear reverse roller 104' against the forward roller 103. This is likely to cause the copy item to be transported obliquely due to the difference of pressing force between the front and rear reverse rollers 104, 104', involving undesirable formation of toner image on the copy item and deterioration of copy image.
To solve this problem, it can be considered to change the arrangement of the front support plate 101 or rear support plate 102 to make the distances L1 and L2 equal to each other. However, there has been a very difficulty in changing of the arrangement of the front or rear support plate 101, 102 because the change of arrangement of the front and rear support plates 101, 102 will unavoidably involve change of arrangement of the other members of the image forming apparatus, such as transport path means, various drive mechanisms.
Also, in the above-mentioned control of drive of the transporting members, the leading edge of the copy item is nipped between the registration roller pair 110 by driving the registration roller pair 110 for the short duration T (t6-t4). After the copy item being nipped, the registration roller pair 110 is stopped at time t6 and restarted at time t7 to transport the copy item to the photosensitive member.
On the other hand, the reverse rollers 104, 104' are kept being rotated in the reverse direction continuously after the nipping duration between times t6 and t7. Accordingly, there is a likelihood that the copy item having been nipped by the registration roller pair 110 is got out of the registration roller pair 110 by the reverse rotation of the reverse rollers 104, 104' and transported backward or upstream again.
More specifically, there are variations in the duration between the detection of leading edge of the copy item by the registration sensor 111 and the arrival of the leading edge at the registration roller pair 110 because of differences in the material and thickness of copy items. This causes the nipping state of copy item between the registration roller pair 110 to be varied. For example, if the leading edge of the copy item is not assuredly nipped by the registration roller pair 110, due to the fact that after time t6, the reverse rollers 104, 104' are kept being rotated in the reverse direction although the forward roller 103 has been stopped, the copy item is transported back by the reverse rotation of the reverse rollers 104, 104'. Accordingly, such backward transport of copy item makes impossible transporting of the copy item to the photosensitive member even when the registration roller pair 110 is restarted at time t7, or disrupts the transport timings to involve a deviation in transfer of toner image onto the copy item.